Think of photography as capturing the light on your subject rather than as a catching a photo of the subject. If you think that way, you need to know how your camera gathers light. There are three ways to control the light that enters the camera, shutter speed, aperture and the ISO setting. Shutter speed controls the amount of time light strikes the sensor and aperture determines the size of the opening that lets the light in. The ISO setting controls the sensitivity of the sensor. The higher the number, the more light sensitive, the sensor is.
When do you use what ISO settings? In low light you may not want to shoot wide open because you want everything in the photo to be tack sharp. You could shoot at a slower shutter speed, but if the subject is moving or could move, a slow shutter speed will create a blur. Furthermore, if you are handholding the camera, a slow shutter speed will also introduce camera shake.
The solution? Increase the ISO setting. Increasing the setting from 100 to 200 doubles the sensor sensitivity and is the equivalent of 1 f-stop. Increasing the setting to 400 doubles it again, 800 doubles it yet again so, changing the ISO from 100 to 800 gains three f-stops.
There is a downside to increasing the ISO setting. Doing so introduces noise into the shot. Some newer digital cameras handle noise pretty well up to ISO 3200 or more. Settings higher than that may create so much noise that the photo becomes unacceptable. Look at the sky in the two images above. The one on the left has a sky full of noise while the one on the right has an acceptable level of noise.
If 1600 ISO doesn’t introduce much noise, why not use that setting all the time? There are times when you want low sensitivity. For example, if you are shooting a waterfall on a bright day and want that silky look created by a slow shutter speed, you wil not be able to get the exposure you want at ISO 1600. If you shoot at a half second and f22, the shot will still be overexposed. Changing the ISO setting from 1600 to 100 will reduce the exposure by four stops. Another way to reduce the light in these situations is to use a neutral density filter. A two stop polarizing filter will also reduce the light.
Whenever you compose that photo, you must think of the light requirements to achieve the shot you want. The camera’s ISO setting is one control at your disposal.